In the prior art, many methods are known for imparting dimensional stability and water repellency to substrates, for example, paper items, fibrous items and building materials such as wood and brick. Typically, materials are coated or impregnated with solutions of silicone, acrylic, urethane, ester, fatty and oily resins or monomers, followed by drying. Of these repellents, silicone repellents are wide spread. In particular, silicone water repellents of the solvent dilution type become the main stream.
However, water repellents of the solvent dilution type generally have a more negative influence of the solvent on the environment than the water dilution type. Also from the standpoints of environmental protection and resource preservation, there is a strong desire to have water repellents which do not use solvents, especially aqueous water repellents of high performance.
While many aqueous water repellents were recently developed, JP-A 1-292089, JP-A 5-156164 and JP-A 5-221748 disclose long term stable emulsions having alkyltrialkoxysilanes emulsified in water. However, these emulsions have several drawbacks since they use alkoxysilanes characterized by very slow hydrolytic reaction. When the emulsion is applied to a material, the material is effectively impregnated therewith, but the silane volatilizes off from the material surface. As a result, the material surface loses water repellency, becomes vulnerable to water wetting, staining and popup by frosting and thus undesirably less durable, and looks milky white on outer appearance.
Aside from the emulsion type mentioned above, JP-A 61-162553, JP-A 4-249588 and JP-A 10-81752 disclose water repellents of homogeneous aqueous solution type.
However, the composition of JP-A 61-162553 lacks storage stability in that rapid polymerization reaction takes place upon dilution with water. The composition must be used within a day after dilution and is thus impractical. The rapid polymerization reaction leads to a molecular weight build-up, which retards impregnation of the material therewith, sometimes leaving wet marks on the material surface.
The composition of JP-A 4-249588 comprising a water-soluble amino group-containing coupling agent and an alkyltrialkoxysilane having a short carbon chain has good storage stability, but poor water repellency probably because only the lower alkyl group contributes to water repellency. Since the amino group-containing coupling agent component is included in excess of the alkylalkoxysilane component as demonstrated by a molar ratio of alkylalkoxysilane component/amino group-containing coupling agent in the range from 0.5/10 to 3/1, there are problems that wet color marks are left on the material surface and paper, fibrous items and wood are substantially yellowed.
JP-A 2000-95868 discloses a method for preparing a composition by first partially hydrolyzing an alkyltrialkoxysilane or alkyldialkoxysilane having a short carbon chain and an amino group-containing alkoxysilane, adding hydrolytic water and an acid to effect further hydrolysis, and finally adding a neutralizing agent. This method is complex. In the first step of effecting hydrolytic reaction on a mixture of the alkylalkoxysilane and the amino group-containing alkoxysilane, the amino group-containing alkoxysilane generally has a higher hydrolytic rate than the alkylalkoxysilane, which becomes a bar against co-hydrolysis, failing to effectively form a co-hydrolytic product. The composition finally obtained by this method is thus unsatisfactory. Treatment of neutral substrates with the composition undesirably imparts poor water repellency.
JP-A 7-150131 discloses the treatment of wood with a composition comprising a salt of an organic or inorganic acid with a basic nitrogen-containing organopolysiloxane, a water repellent substance and water. This composition, however, has the problems of insufficient water repellency and storage instability.
JP-A 55-133466 and JP-A 55-133467 disclose a composition obtained by hydrolyzing an alkylalkoxysilane, an amino group-containing alkoxysilane, an epoxy group-containing alkoxysilane and a metal-metalloid salt with water. The treatment of substrates with the composition minimizes yellowing. However, since amino groups are blocked by the reaction of amino groups with epoxy groups, the composition becomes so difficultly soluble in water that it cannot be used as an aqueous treating agent. The amino blocking also restrains the adsorption of the composition to substrates so that the composition cannot be used for the treatment of substrates.
To solve the above problems, we proposed in JP-A 9-77780 a composition comprising the co-hydrolyzate of an alkylalkoxysilane having 7 to 18 carbon atoms, an alkoxy group-containing siloxane and an amino group-containing alkoxysilane. Despite the use of long chain alkyl silane, the composition provides substrates with weak water repellency. When paper, fibrous items and wood are treated with the composition, somewhat noticeable yellowing occurs.
Proposed in JP-A 10-81752 is a binder composition which is stable in an alkaline region. Due to a substantial amount of amino group-containing alkoxysilane used therein, this composition had many problems including insufficient water repellency as an agent for treating non-alkaline substrates, wet color left on the treated material, and substantial yellowing.
Accordingly, all the water repellents described above are seldom regarded as performing satisfactorily for the treatment of substrates, especially neutral (weakly acidic to weakly alkaline) substrates.
On the other hand, housing members available at present include plywood members which are often used as bearing wall members, structural floor sheathing members, and roof sheathing members, and laminated veneer lumber which are often used as two-by-four members and Japanese traditional wooden framework members.
It has heretofore been possible to produce plywood and laminated veneer lumber from a useful wood raw material having excellent properties which is selected for a particular purpose or application from among wood raw materials having relatively good properties, for example, tropical timber. Due to the depletion of wood resources, it is not always possible under the currently prevailing circumstances to use only a wood raw material having excellent properties. Now that the regulation of insuring and promoting the quality of houses and buildings has been enforced, the quality demand to housing members is and will be increasing. It is forecasted that the future need is to produce plywood or laminated veneer lumber which are less expensive, have good physical properties and impose a less load to the environment upon discarding.
These facts suggest that with the progress of depletion of wood resources, the preparation of wooden panels from a wood material having excellent properties as the raw material is not always possible. In particular, plywood and laminated veneer lumber products from a typical forested tree, Radiate pine (Pinus Radiata D. DON) as the raw material have not been widespread because of problems including dimensional changes, warping and mildewing due to their high water and moisture absorptive properties.
One conventional approach use to solve these problems is to apply emulsions of acrylic water repellents or paraffinic water repellents. However, a blocking problem often occurs when these water repellents are applied to plies and dried and the plies are piled up. This problem precludes widespread use in practical applications.
For the preparation of wooden fiberboards, wet and dry methods are known in the art. In either method, sheet-shaped articles of wooden fibers obtained by paper machining or sheeting are generally heat compression molded by means of a hot press or similar equipment. The heat compression molded fiberboards, immediately after exiting from the hot press, are cooled and piled up in a cooling equipment of the elevator or wicket type.
In the method of preparing such fiberboards, it is common to use adhesives comprising formaldehyde-containing resins such as urea-formaldehyde resins, melamine-formaldehyde resins, and phenol-formaldehyde resins, alone or in admixture. At the same time, various waxes are used in the adhesives to impart water resistance to the fiberboards, for example, so-called synthetic waxes such as acrylic waxes, polyethylene waxes synthesized from polyethylene having a low degree of polymerization or acid modified products thereof, Fischer-Tropsch wax synthesized from carbon monoxide and hydrogen, and amide waxes synthesized from various fatty acids and ammonia or amine; petroleum base waxes such as paraffin wax and microcrystalline wax, and mineral waxes such as montan wax, ozokerite and ceresine.
Also, since the wooden fiberboards are molded under pressure, they experience substantial dimensional changes due to absorption and release of moisture or water after the molding. When they were actually used in houses, problems frequently occurred. Then studies have been made to improve the water resistance of wooden fiberboards for the purpose of improving the dimensional stability of wooden fiberboards. Besides the above-mentioned exemplary solution of wax addition, it has also been proposed to use isocyanate base adhesives having high water resistance, to carry out heat or steam treatment at high temperature (150 to 200° C.), and to carry out chemical treatment such as formalization.
However, the use of the above-mentioned waxes as means for improving the water resistance of wooden fiberboards generally tends to compromise the strength performance of fiberboards such as bending strength and internal bond strength. The use of isocyanate base adhesives has been under study and actual use as mentioned above, although these adhesives are very expensive as compared with urea-formaldehyde base resins and melamine-formaldehyde base resins, and so toxic that meticulous handling and strict management are required on their use as well as the new addition or modification of a safety security step.
Among the water resistance-ameliorating measures, the use of isocyanate base adhesives having high water resistance has the problems of expensiveness and meticulous handling and strict management on their use as described just above; and the heat treatment at high temperature (150 to 200° C.) has the drawback of an increased cost required to provide the high temperature and further raises the problem of requiring an extra step of increasing the water content of fiberboards, which has been once decreased to nearly absolute dryness during the treatment, to a water content (5 to 13%) which is acceptable on practical use. The steam treatment has the problems that the processing equipment is costly, and the running cost is very high. The chemical treatment such as formalization is very costly in itself and in the case of formalization, the increased amount of formaldehyde released is a problem.
Further, a method involving applying a surface modifier to a sheet-shaped member of wood fibers as by spraying, followed by heat compression molding, as disclosed in JP-A 2001-260104, has the problem that since excessive portions are cut away in finishing the member into a product, water can penetrate into the member through end faces so that member is readily swollen.
As discussed above, the prior art methods of improving water resistance have problems on practical usage in that reagents such as waxes are cost effective, but induce a decline of the strength performance of wooden fiberboards, the above-described adhesives, heat treatment, steam treatment and chemical treatment are effective for improving water resistance, but invite cost increases; that is, any of these measures fails to satisfy both of these requirements.
An object of the invention, which has been made in consideration of the above-mentioned circumstances, is to provide an aqueous water repellent for the treatment of substrates of materials originating from lignocellulose or the like which is improved in impregnation of the substrates therewith and imparts dimensional stability and water repellency to the treated substrates, and a method for preparing the same.
Another object of the invention is to provide a method for preparing modified plywood or modified laminated veneer lumber, which method can render plywood or laminated veneer lumber termite-proof, rot-proof, mildew-proof, water resistant, moisture resistant and dimensional stable and thus accomplish the desired performance without detracting from the lightweight advantage thereof.
A further object of the invention is to provide a method for preparing wooden fiberboards, in which wooden fiberboards endowed with water resistance, durability and strength performance can produced at a high productivity and low cost.